Fix MULTIDRIVEN in generates (#7709)

src/V3Undriven.cpp

@@ -29,6 +29,7 @@
 
 #include "V3Stats.h"
 #include "V3UndrivenCapture.h"
+#include "V3Width.h"
 
 #include <vector>
 
@@ -369,6 +370,7 @@ class UndrivenVisitor final : public VNVisitorConst {
     bool m_inProcAssign = false;  // In procedural assignment
     bool m_inFTaskRef = false;  // In function or task call
     bool m_inInoutOrRefPin = false;  // Connected to pin that is inout
+    bool m_inSelLhs = false;  // Iterating the fromp of an AstSel (a partial-bit write target)
     const AstNodeFTask* m_taskp = nullptr;  // Current task
     const AstAlways* m_alwaysp = nullptr;  // Current always of either type
     const AstAlways* m_alwaysCombp = nullptr;  // Current always if combo, otherwise nullptr
@@ -473,8 +475,11 @@ class UndrivenVisitor final : public VNVisitorConst {
                     entryp->usedBit(lsb, nodep->width(), varrefp);
             }
         } else {
-            // else other varrefs handled as unknown mess in AstVarRef
-            iterateChildrenConst(nodep);
+            // skip over static longest static prefix
+            iterateConst(nodep->lsbp());
+            VL_RESTORER(m_inSelLhs);
+            m_inSelLhs = !V3Width::selectNonConstantRecurse(nodep->lsbp(), /*inSel=*/true);
+            iterateConst(nodep->fromp());
         }
     }
     void visit(AstNodeVarRef* nodep) override {
@@ -530,7 +535,7 @@ class UndrivenVisitor final : public VNVisitorConst {
                 if (entryp->isDrivenWhole() && !m_inBBox && !VN_IS(nodep, VarXRef)
                     && !VN_IS(nodep->dtypep()->skipRefp(), UnpackArrayDType) && !sameFileLine
                     && !entryp->isUnderGen() && otherWritep && !entryp->isFtaskDriven()
-                    && !ftaskDef
+                    && !ftaskDef && !m_inSelLhs
                     && !nodep->varp()->fileline()->warnIsOff(V3ErrorCode::MULTIDRIVEN)) {
                     const bool otherWriteIsStaticInit
                         = nodep->varp()->hasUserInit() && otherWritep == entryp->initStaticp();

src/V3Width.cpp

@@ -9948,7 +9948,7 @@ class WidthVisitor final : public VNVisitor {
     }
     void checkForceReleaseLhs(AstNode* nodep, AstNode* lhsp) {
         // V3Force can't check as vector may have expanded, or propagated constant into index
-        if (AstNode* const selNodep = selectNonConstantRecurse(lhsp))
+        if (AstNode* const selNodep = V3Width::selectNonConstantRecurse(lhsp))
             nodep->v3error((VN_IS(nodep, Release) ? "Release"s : "Force"s)
                                + " left-hand-side must not have variable bit/part select "
                                  "(IEEE 1800-2023 10.6.2)\n"
@@ -9956,34 +9956,6 @@ class WidthVisitor final : public VNVisitor {
                            << selNodep->warnOther() << "... Location of non-constant index\n"
                            << selNodep->warnContextSecondary());
     }
-    AstNode* selectNonConstantRecurse(AstNode* nodep, bool inSel = false) {
-        // If node has a non-constant select, return that select
-        AstNode* resultp = nullptr;
-        if (AstNodeSel* const anodep = VN_CAST(nodep, NodeSel)) {
-            resultp = selectNonConstantRecurse(anodep->fromp(), inSel);
-            if (resultp) return resultp;
-            resultp = selectNonConstantRecurse(anodep->bitp(), true);
-        } else if (AstSel* const anodep = VN_CAST(nodep, Sel)) {
-            resultp = selectNonConstantRecurse(anodep->fromp(), inSel);
-            if (resultp) return resultp;
-            resultp = selectNonConstantRecurse(anodep->lsbp(), true);
-        } else if (AstNodeVarRef* const anodep = VN_CAST(nodep, NodeVarRef)) {
-            if (inSel && !anodep->varp()->isParam() && !anodep->varp()->isGenVar()) return anodep;
-        } else {
-            if (AstNode* const refp = nodep->op1p())
-                resultp = selectNonConstantRecurse(refp, inSel);
-            if (resultp) return resultp;
-            if (AstNode* const refp = nodep->op2p())
-                resultp = selectNonConstantRecurse(refp, inSel);
-            if (resultp) return resultp;
-            if (AstNode* const refp = nodep->op3p())
-                resultp = selectNonConstantRecurse(refp, inSel);
-            if (resultp) return resultp;
-            if (AstNode* const refp = nodep->op4p())
-                resultp = selectNonConstantRecurse(refp, inSel);
-        }
-        return resultp;
-    }
 
     //----------------------------------------------------------------------
     // METHODS - special type detection
@@ -10139,3 +10111,28 @@ AstNode* V3Width::widthGenerateParamsEdit(
     // No WidthRemoveVisitor, as don't want to drop $signed etc inside gen blocks
     return nodep;
 }
+
+AstNode* V3Width::selectNonConstantRecurse(AstNode* nodep, bool inSel) {
+    // If node has a non-constant select, return that select
+    AstNode* resultp = nullptr;
+    if (AstNodeSel* const anodep = VN_CAST(nodep, NodeSel)) {
+        resultp = selectNonConstantRecurse(anodep->fromp(), inSel);
+        if (resultp) return resultp;
+        resultp = selectNonConstantRecurse(anodep->bitp(), true);
+    } else if (AstSel* const anodep = VN_CAST(nodep, Sel)) {
+        resultp = selectNonConstantRecurse(anodep->fromp(), inSel);
+        if (resultp) return resultp;
+        resultp = selectNonConstantRecurse(anodep->lsbp(), true);
+    } else if (AstNodeVarRef* const anodep = VN_CAST(nodep, NodeVarRef)) {
+        if (inSel && !anodep->varp()->isParam() && !anodep->varp()->isGenVar()) return anodep;
+    } else {
+        if (AstNode* const refp = nodep->op1p()) resultp = selectNonConstantRecurse(refp, inSel);
+        if (resultp) return resultp;
+        if (AstNode* const refp = nodep->op2p()) resultp = selectNonConstantRecurse(refp, inSel);
+        if (resultp) return resultp;
+        if (AstNode* const refp = nodep->op3p()) resultp = selectNonConstantRecurse(refp, inSel);
+        if (resultp) return resultp;
+        if (AstNode* const refp = nodep->op4p()) resultp = selectNonConstantRecurse(refp, inSel);
+    }
+    return resultp;
+}

src/V3Width.h

@@ -31,6 +31,7 @@ public:
     static void width(AstNetlist* nodep) VL_MT_DISABLED;
     static AstNode* widthParamsEdit(AstNode* nodep) VL_MT_DISABLED;
     static AstNode* widthGenerateParamsEdit(AstNode* nodep) VL_MT_DISABLED;
+    static AstNode* selectNonConstantRecurse(AstNode* nodep, bool inSel = false) VL_MT_DISABLED;
 
     // For use only in WidthVisitor
     // Replace AstSelBit, etc with AstSel/AstArraySel

test_regress/t/t_lint_always_ff_multidriven_genif.py

@@ -0,0 +1,16 @@
+#!/usr/bin/env python3
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# This program is free software; you can redistribute it and/or modify it
+# under the terms of either the GNU Lesser General Public License Version 3
+# or the Perl Artistic License Version 2.0.
+# SPDX-FileCopyrightText: 2026 Wilson Snyder
+# SPDX-License-Identifier: LGPL-3.0-only OR Artistic-2.0
+
+import vltest_bootstrap
+
+test.scenarios('linter')
+
+test.lint()
+
+test.passes()

test_regress/t/t_lint_always_ff_multidriven_genif.v

@@ -0,0 +1,28 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain.
+// SPDX-FileCopyrightText: 2026 Wilson Snyder
+// SPDX-License-Identifier: CC0-1.0
+
+module t(input wire clk);
+
+  parameter int max_ports = 4;
+
+  logic [max_ports-1:0][7:0] x;
+
+  localparam logic [max_ports-1:0] use_a = 4'b0101;
+
+  for (genvar port = 0; port < max_ports; port++) begin : g
+    if (use_a[port]) begin : g_a
+      always_ff @(posedge clk) x[max_ports - port - 1] <= "a";
+    end else begin : g_b
+      always_ff @(posedge clk) x[max_ports - port - 1] <= "b";
+    end
+  end
+
+  initial begin
+    $write("*-* All Finished *-*\n");
+    $finish;
+  end
+
+endmodule